The list of Mozilla extensions is far too long for this presentation,
and new ones are showing up all the time.. A good place to look
for them is on Mozdev.org.
Some examples of what extensions can do:
Flash blocking: implements "Flash click to view", so that when
you go to a page with embedded flash, instead of seeing the animation
you see a static button, and the flash displays only if you press the
Tabbed Browser Extensions: has an option to make new
windows open in a new tab instead; allows for reordering of tabs by
dragging, and for renaming of tabs; various functions for ordering and
saving tab groups.
Optimoz: implements mouse gestures. Sweep the mouse right
to go forward and left to go back. Lots of other gestures.
Invent your own!